While there is no exact definition of a small uterus, one that is about 25% smaller than average should still be able to support a pregnancy. The average uterus is 3 inches (7.5 cm) long, 2 inches (5 cm) wide, and 1 inch (2.5 cm) deep. Symptoms of a small uterus include irregular or no periods and abdominal pain. However, many women are unaware of uterine abnormalities as there may be no symptoms at all.
A healthy uterus is pear-shaped, hollow, and lined with thick muscular walls. There are two fallopian tubes that are connected to an ovary filled with eggs that enter the uterus on either side at the top. A uterus that differs in structure from the norm is considered an abnormality that may or may not lead to difficulties in conception and pregnancy. The normal-sized or smaller-than-average uterus stretches and grows when stimulated by hormones released during pregnancy. Only when size is an indication of other conditions can a small uterus cause problems.
The process of development of the female reproductive system begins in the embryo with the primordial müllerian ducts. These are paired tubes that differentiate to form the fallopian tubes, uterus, and vagina in the girl. A very rare condition that affects one woman in many thousands, called uterine agenesis, is a Mullerian duct anomaly. Uterine agenesis results in a very small uterus or none at all and usually presents with vaginal agenesis, which is a missing or incomplete vagina. In this case, surrogacy is almost always the only solution for women to have a child.
Another anomaly of the müllerian duct leads to a unicornuate uterus. This is usually only half the size of a normal uterus and there is only one fallopian tube instead of two. As long as this is healthy and developed, the woman should be able to conceive, but she will need careful monitoring during pregnancy and delivery, as having a unicorn uterus significantly increases the risk of pregnancy loss and preterm labor.
There are many ways to diagnose a small uterus. The most common test used first is the 2D ultrasound, which is used by most OB/GYNs. This is enough to indicate a problem, but the images are often not sharp enough for a diagnosis. Other tests include 3D ultrasound, magnetic resonance imaging (MRI), or a minor operation called a laparoscopy.